The present invention relates to fuel cells, and more particularly to fuel cells constructed of stacked plate components. More particularly, the present invention relates to fuel cells containing enhanced flow electrodes for fuel and/or air.
The invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell.
Although particular embodiments are applicable to conventional co-fired solid electrolyte fuel cell apparatus, the present invention is particularly useful when utilizing non-cofired solid oxide electrolyte fuel cells, preferably planar fuel cells, that contain a stack of multiple assemblies. Each assembly comprises a solid electrolyte disposed between a cathode and an anode, being bounded by separators, which contact the surfaces of the electrodes opposite the electrolyte.
The fuel cell operates by conducting ions through the electrolyte. For solid oxide fuel cells in particular, oxygen or air is introduced at the cathode, and ionization of oxygen occurs at the cathode/electrolyte surface. The oxygen ions move across the gas non-permeable electrolyte to the anode interface, where it reacts with the fuel flowing into the anode at the anode/electrolyte interface, releasing heat and supplying electrons to the anode. Distribution of the air and fuel reactants is typically performed by a manifold assembly within the fuel cell apparatus.
Conventionally, each reactant is supplied through a flow conduit to the appropriate electrode, and distribution to the electrode/electrolyte interface is accomplished by internal porosity and/or grooved channels.
Minh, U.S. Pat. No. 5,256,499, discloses a monolithic fuel cell having an integrally formed manifold constructed by corrugations formed within the anode and cathode with aligned ribs and columns arranged to force fuel and oxidant along aligned pathways. Reactants are fed from the sides of the fuel cell and travel along these pathways.
Hsu, U.S. Pat. No. 5,747,485, discloses a conductor plate for a solid oxide fuel cell with ridges extending therefrom. These ridges form grooves used to channel reacting gases out of the cell.
Datta, U.K. Patent No. 2,219,125A discloses an electrolyte with a three-dimensional groove arrangement used to control hot spots within the electrolyte block.
Hsu, Minh and Datta employ external manifolding and rectangular geometries driving the reactants from one side of the cell to the other. Despite the use of channels, reactants entering from a single side of the cell deplete as they travel across the cell. Further, when reactants are fed externally from more than one side, the flows converge creating localized areas of increased reaction. The increased number of reactions generates an undesirable thermal gradient, which can damage the cell.
Moreover, Hsu, Minh and Datta employ grooves of uniform cross section along the length of these grooves. These grooves are essentially pathways within the cell, and fail to control gas flow rate or pressure distribution. The flow rate is controlled at its source and not tailored or controlled within the cell.
In fuel cells which have their anode fuel-exit edges exposed to an oxidizing environment, any anode local exit regions having low fuel mixture velocities may allow oxygen back diffusion into the cell stack, causing premature combustion and loss of active anode area. The electrochemical processes inherent in the fuel cell""s operation become less effective and performance suffers.
Custom flow pattern design is desirable to achieve substantially uniform reactant concentration distribution within the cell and from cell to cell within a stack, which also helps minimize unnecessary and undesirable thermal gradients within the cell.
It is an object of the present invention, therefore, to provide a compact, centrally fed radial fuel cell utilizing micro-channels to tailor the flow distribution of reacting gases within the fuel cell and amongst all the cells in a stack.
It is another object of the present invention, to provide a compact fuel cell utilizing variable cross-section micro-channels to tailor the flow, pressure, and velocity distribution of reacting gases within the fuel cell and amongst all the cells in a stack.
It is a further object of the present invention to provide an enhanced flow electrode produced by simple scalable production techniques.
We have found that micro-channels integrated within the electrode structure can be formed in a compact fuel cell. Integrated micro-channels minimize the complexity of stack components. Channels of smaller dimension than those existing in the prior art can be manufactured by a variety of techniques. Using these techniques, flow and pressure distribution can be customized and controlled through the channel design, enhancing reactant distribution to the cell. It has further been found that a fuel cell apparatus employing a network of micro-channels can improve overall cell reactant balance through controlled pressure distribution. It has further been found that employing controlled flow and pressure in a compact integrated device results in an apparatus exhibiting improved volumetric power density and efficiency.
The present invention therefore provides an electrochemical apparatus comprising at least one cell, wherein the cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, with at least one separator between adjacent cells contacting the surface of one of the electrodes opposite the electrolyte; wherein at least one electrode of the cell defines a variable cross-section micro-channel pattern, wherein this pattern serves to distribute the flowing gas uniformly within the electrode, regulates the pressure drop of this gas, and also creates preferred local gas velocities, especially where the gas exits the electrode.
The present invention further provides an electrochemical apparatus comprising at least one cell, having a solid electrolyte disposed between an oxygen electrode and a fuel electrode; and at least one separator contacting the surface of one of the electrodes opposite the electrolyte. In one embodiment, at least one separator preferably defines a micro-channel pattern; wherein the micro-channel pattern narrows towards the cell rim, such that gas flowing out the rim is accelerated.
The micro-channel is preferably a small size, on the order of about 0.5 millimeter or less, such that the micro-channel can be defined within at least one electrode or separator by low-cost manufacturing techniques.
The present invention also provides an electrochemical apparatus comprising an electrode defining a pattern of micro-channels for directing the flow of reactant; wherein the cross sectional area of the micro-channels is varied along the micro-channel length.
The present invention also provides an electrochemical apparatus comprising a plurality of cells forming a stack; each cell within the stack has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, with at least one separator contacting the surface of one of the electrodes opposite the electrolyte. In substantially each of these cells, at least one electrode defines a variable cross-section micro-channel pattern.
The present invention also provides an electrochemical apparatus comprising at least one cell having a solid electrolyte disposed between an oxygen electrode and a fuel electrode, and at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein at least one electrode or the electrolyte or the separator surface has a plurality of columns extending therefrom; said columns defining variable cross-section micro-channels therebetween.
The present invention also provides an electrochemical apparatus comprising at least one circular cell having a cell rim; said cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer; at least one separator layer contacting the surface of one of the electrodes opposite the electrolyte; wherein each of the layers define at least one air hole and at least one fuel hole and wherein the respective holes in each layer are registrable with one another and define generally central internal air and fuel manifolds; wherein at least one layer has a plurality of circular columns extending longitudinally outwardly from the respective air or fuel manifold, defining a micro-channel pattern. Preferably, the columns are arranged in radially expanding rows; and an increasing number of columns extend from said at least one layer in each of said rows, such that said columns define a variable cross-section micro-channel that narrows toward the cell rim.
The present invention also provides an electrochemical apparatus comprising at least one fuel cell, wherein the cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, and at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein the cell defines at least one air manifold and at least one fuel manifold located substantially centrally within the cell; and at least one of the electrodes defines a micro-channel pattern.
The present invention further provides, in a process for the fabrication of a solid oxide fuel cell comprising at least one cell having a cell rim, wherein said cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer, and at least one separator layer contacting the surface of one of the electrodes opposite said electrolyte; wherein each of the layers define at least one air hole and at least one fuel hole and wherein the respective holes within each layer are registerable with one another and define generally central internal air and fuel manifolds; the improvement including providing reactant micro-channels in at least one layer, said micro-channels having a width of not more than about 0.5 mm.
The micro-channel patterns may be fabricated by a variety of known fabrication methods. One preferred method is the use of mechanical pressing.